There are many circumstances wherein it is desirable to utilize small chips of cellulosic fibrous material (typically wood) in the production of paper pulp. However heretofore it has been difficult to handle small chips utilizing conventional equipment. Small chips require different equipment and processing than regular size chips, or sawdust. A small chip may be defined as one having a smallest dimension of about 1-5 mm, typically roughly about 2 mm. The length of the chips is not critical, rather it is their minimum dimension (thickness) that is important.
According to the present invention it is possible to properly treat small chips to produce paper pulp. Small chips allow a better penetration with the treatment chemicals, resulting in a more uniform cooking process. Small chips also provide a more uniform Kappa distribution which makes it possible to cook to lower Kappa numbers without losing strength. Further, small chips can be made up of poorer raw material than conventional large chips, including waste from saw mills, processing facilities, and the like that is not in the pulverized form of sawdust.
One of the keys to being able to handle the small chips according to the invention is the ability to treat the chips without liquor circulations. Liquor circulations, which are standard in conventional impregnation vessels and digesters, are not feasible when treating small chips. Therefore according to the present invention it is necessary to move the chips through the treatment liquid, rather than circulating the treatment liquid.
According to the invention, chips are first steamed in a conventional steaming vessel and then passed through a low pressure feeder into an impregnation vessel with an interior conveying means. Preferably the impregnation vessel is disposed at an angle of between about 30.degree.-60.degree. to the horizontal and includes an endless belt with paddles extending therefrom which moves within the vessel. Sulfur containing liquor is added to the impregnation vessel, and the conveyor moves the chips through the liquor to a second lower pressure feeder at the top of a vertical digester.
At the top of the digester, steam is supplied to the chips to heat them to cooking temperature (e.g. about 150.degree.-180.degree. C.). The chips move in the slurry of white liquor down through the digester to be discharged from the bottom. The digester has smooth interior walls, being devoid of screens and circulation loops.
At the bottom of the digester, the slurry has a consistency of about 10 to 20%, and typically it is diluted--as with black liquor--to a consistency of about 10% or less. The chips then pass through a feeder without significant pressure reduction to a pressure diffuser or like pressure washing vessel, where the chips are subjected to a first wash. The liquor withdrawn from the first pressure diffuser is the black liquor that many be used for diluting the chips at the bottom of the digester, and also a part may be fed to a flash tank and the heat recovered therefrom to heat cooking liquor being supplied to the impregnation vessel.
From the top of the first pressure diffuser the paper pulp passes through a flow control valve and then to the top of a hot alkali extraction tank, which also has no circulation loops. From the bottom of that tank it goes to a second pressure diffuser where it is subjected to a second wash, with the liquor withdrawn from the second pressure diffuser being the wash liquor for the first pressure diffuser. The pulp withdrawn from the top of the second pressure diffuser may then be subjected to oxygen delignification, bleaching, or other additional treatment steps.
By practicing the present invention it is possible, in an energy efficient manner, to effectively produce paper pulp from chips having a smallest dimension of about 1 to 5 mm (typically about 2 mm). While the invention is particularly desirable for the treatment of small chips, however, it is to be understood that various aspects of the method and apparatus according to the invention also can be applied to conventional chips.